Say nothing. Accept with thanks.
Sell the book on ebay. (or return back to store where bought, if known)
Buy the voucher
If SIL asks if he liked the book, say "no,
he I sold it on ebay to buy a music shop voucher...you know he doesn't I don't read much and much prefers music".
Hopefully then it may sink in.
agree with this. But if it's that big a deal to your DH, let him do the legwork. It's not your call. If he would prefer to sell the book and use the money towards buying music, let him arrange to do that. And he should be the one to tell his sister, not you.
You've told her what he would like. It's not your responsibility to "fix" this.